Applications of group III nitride semiconductor devices and in particular, devices for emitting or receiving light having a wavelength of 360 nm or less in the ultraviolet or deep ultraviolet region which use AlxGa1-xN (0<x≦1) substrates have been anticipated for some time in the field of medical treatments or precision machining.
With respect to such devices for emitting or receiving light in the ultraviolet or deep ultraviolet region, in those cases where either a template substrate in which a GaN layer is laminated on top of a substrate composed of a single crystal of different kinds such as sapphire or SiC, or an independent GaN substrate is used in a conventional manner, light emitted from a light emitting layer is absorbed by the GaN layer, which is a problem.
Further, in those cases where an AlGaN layer having a high ratio of Al composition is deposited on a GaN layer, cracks are formed due to the differences in the lattice constant and the coefficient of thermal expansion, thereby causing deterioration in the device characteristics.
In order to solve these problems, it is necessary to eliminate the light absorption and to enhance the efficiency for emitting or receiving light by using an AlGaN substrate having an adequate composition for transmitting light with a wavelength which can be emitted or received by the devices. At the same time, it is necessary to suppress the occurrence of cracks or dislocations and to improve the crystal quality by reducing the differences in the lattice constant and the coefficient of thermal expansion between the light-emitting and receiving layers. However, the quality of AlxGa1-xN (0<x≦1) crystals achieved to date has not been satisfactory. Since an AlxGa1-xN (0<x≦1) crystal having a particularly high molar fraction of AlN exhibits characteristics that are close to those of AlN such as a high melting point and a low vapor pressure as compared to a GaN crystal, it has been difficult to achieve a satisfactory level of crystal growth.
Incidentally, as a document disclosing a method for manufacturing a GaN substrate formed by an epitaxial lateral overgrowth (ELO) process in order to improve the crystallinity, for example, the following Patent Document 1 is known.
That is, according to Patent Document 1, there is disclosed a method for manufacturing a GaN-based semiconductor element in which a first GaN-based semiconductor layer is formed on top of a sapphire substrate, followed by formation of a mask pattern composed of a silicon oxide film (SiO2) or a silicon nitride film (SiN) on top of this first GaN-based semiconductor layer, and a second GaN-based semiconductor layer is then formed by the ELO process using the mask pattern. As a result, according to the disclosure, a GaN-based semiconductor layer in which the threading dislocation in the vertical direction is suppressed can be achieved.
Although it may be possible to employ the ELO process to improve the crystal quality of AlxGa1-xN (0<x≦1), when an AlN-based semiconductor layer is formed by the conventional ELO process, the following problems arise. That is, as shown in FIG. 4, an AlN semiconductor layer 102 serving as a group III nitride layer is formed on top of a sapphire substrate 101, followed by formation of a mask pattern 103 composed of a silicon oxide film (SiO2) or a silicon nitride film (SiN) on top of this AlN semiconductor layer 102, and ELO layers 104 composed of AlxGa1-xN (0<x≦1) are grown by the ELO process using the mask pattern 103 from regions R that are not covered by the mask pattern 103.
Here, in those cases where the ELO layer 104 is composed of a GaN-based semiconductor layer as in the case disclosed in Patent Document 1, since a GaN crystal does not grow on top of the mask pattern 103 composed of a silicon oxide film (SiO2) or a silicon nitride film (SiN), a GaN layer grown from the region R that is not covered by the mask pattern 103 is grown above the mask pattern 103 in the transverse direction. Accordingly, since crystal defects are prevalent in the growth direction, it is thought that the threading dislocation in the vertical direction is suppressed above the mask pattern 103.
However, when those compositions that include Al as a component such as AlxGa1-xN (0<x≦1) are grown by the ELO process, because a polycrystal 105 is grown on an upper surface 103a of the mask pattern 103 composed of a silicon oxide film (SiO2) or a silicon nitride film (SiN), the ELO layer 104 cannot grow above the mask pattern 103 in the transverse direction. As a result, the threading dislocation in the vertical direction cannot be suppressed, making it impossible to improve the quality of an AlxGa1-xN (0<x≦1) crystal by employing the ELO process.    [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. Hei 11-251632